Hulagu Khan, commander of the Mongols in the Middle East and founder of the Persia-based Il-Khanate, was the grandson of Genghis Khan and brother to both China's Kublai Khan, and to another Kha-Khan ("Great Khan", the title carried by the overlord of the entire Mongol Empire), Khan Möngke. At its peak, the realm Hulago created included Iran, Iraq, Kurdistan; and parts of Turkey, Syria, and Jordan.
The sack of Baghdad culminated the initial phase of the Mongol attempt to conquer the Middle East; begun with Genghis Khan's conquest of Khwarezmia in 1221. A project abandoned after Genghis Khan's death, his grandson took up the task; supported by perhaps the largest Mongol army ever assembled: by order of his brother, Great Khan Möngke, two-tenths of the empire's fighting men were allocated to the task. Not less than 100,000 fighting men formed Hulago's horde; and likely many more than that.
Hulago opened the campaign by attacking Alamut; the chief stronghold of the feared Asasiyun (Assassins) . This fortress citadel, thought at the time to be impregnable, lay in the mountains of Iran, about 60 miles from modern Tehran. Seeing the "handwriting on the wall", the Assassins surrendered on condition their lives were spared. But Alamut was destroyed, and with it the power of the Assassin cult, which had terrorized the Middle East since the 11th century.
Baghdad was then the ancient seat of the Abbasid Caliphate; a secular and religious authority within Islam that dated back to the 8th century. Established after the overthrow of the original Caliphate of the Umayyads in 750, the first Abbasid Caliph (which title means “Successor” to Mohammed) had built Baghdad as his new capital. For centuries, Baghdad was the power-center of Islam in the world. Though secular power had since the 11th century rested in the hands of Turkish Sultans; the Caliph was still the ultimate religious authority within Islam. Though schismatic Caliphates had contested Abbasid authority from time-to-time in Spain, Morocco and Egypt, the Abbasid Caliphate in Baghdad was the oldest and most recognized throughout the world.
Baghdad at its height in the 11th century had claim to being one of (if not the) largest cities in the world; boasting a population of between 1.2 million and 2 million souls. The city had a uniquely Persian design and flavor: unlike in Greek and Roman engineering tradition, where cites are laid out in a rectangular grid, the Persians built cities in a circular pattern, all streets radiating out from a central hub. At the center of Baghdad was the Golden Gate Palace; residence of the caliph and center of his administration. Surmounting the building was a 39 meter-high green dome; one of the largest in the world in its days (18 meters higher than Jerusalem's Dome of the Rock; though fully 15 meters less than the lofty dome of Hagia Sophia.
In the face of imminent Mongol invasion, the foolish Caliph Al-Musta'sim took no steps to call upon allies, raise additional troops, or strengthen his capital. Hulago's massive army reached Baghdad on January 29th, defeating the inadequate Abbasid army outside the walls (breaking dykes to flood their position, drowning many of the Caliph's army in the process). The siege was brief by the standards of the time. The Mongols wheeled up siege engines and catapults; and Baghdad was subjected to the "endless storm", in which warriors attacked day-and-night, in shifts, till the walls were carried. By February 5 the Mongols controlled a stretch of the wall. The Caliph attempted too late to negotiate, but was refused. On February 10th the city surrendered. Baghdad was subject to a week of sack and pillage. Much of the burgeoning population was put to the sword, the gutters and canals of the city running red. Before they were done, the Mongols destroyed large sections of the city; gratuitously ruining canals and dykes forming the city's irrigation system and water supply. Baghdad would never recover its former glory.
The last Caliph was put to death shortly after. Because many of Hulagu's soldiers were themselves Muslims; and because to them it was sacrilege to shed the Caliph's “holy” blood, Hulago had the Caliph wrapped in a Persian rug and thrown into the street. The Khan's horsemen then rode over the rug, crushing the Caliph to pulp within.An alternative story, relayed by Marco Polo, is that Hulagu Khan found the caliph's great storeroom filled with treasure; which could have been spent on the defense of his realm. The Khan had Al-Musta'sim locked him in his treasure room without food or water, telling him "eat of thy treasure as much as thou wilt, since thou art so fond of it."
The fall of the Abbassid Calphate ended any central authority in Islam (though later Muslim leaders from time-to-time have claimed such authority; most notably the Ottoman Turkish Sultans). To this day, no such universally recognized central authority exists. In dealing with the Islamic world, we face this problem daily; as every Imam has the right to issue fatwas as his own conscience dictates, without reference or recourse to a higher authority.
The stated goal of our foes, ISIS and Al Qaeda, is to recreate the lost Caliphate; that jihad against the West can continue under a united Islamic World.